1. Field of the Invention
The present invention relates to a supercharged internal combustion engine and, more particularly, to an internal combustion engine equipped with a super-charger which is controlled in accordance with the operating conditions of the engine.
2. Description of the Prior Art
Engine superchargers include those of a mechanically driven type and exhaust gas pressure type. A supercharger of the second type is so-called "turbo supercharger". The conventional supercharged internal combustion engine is so designed as to be supercharged over the entire operational range from throttle part-open to full-open engine operating conditions. The engine is provided with either a bypass exhaust passage or a bypass intake passage to avoid unduly high supercharging pressure so that an increased engine output is obtained in the throttle full-open engine operating condition only.
Japanese Patent Laid-Open Publication No. 52-18517 laid-open on Feb. 12, 1977 for public inspection discloses a supercharged internal combustion engine which is so designed as to be supercharged not in throttle-part-open engine operating condition but in throttle-full-open engine operating condition. In the throttle-part-open engine operating condition, an intake passage is closed by a control valve so that pressurized air from a supercharger is released into the atmosphere through a release valve, while the engine is charged with the atmospheric pressure through an atmospheric air passage. In the throttle-full-open engine operating condition, however, the control valve closes the atmospheric air passage and opens the intake passage so that the pressurized air produced by the supercharger is introduced into the engine. The release valve is closed at this time. The valves are controlled by signals representative of air pressure in the intake manifold of the engine.
However, these conventional superchargers have various problems and shortcomings.
More specifically, the first-mentioned way of supercharging, i.e. the supercharging over the entire range of engine operation, is not advantageous in that the intake air is restricted or throttled by the throttle valve during the throttle-part-open engine operation to render the supercharging itself meaningless. In addition, especially in the exhaust-pressure type supercharging in which the exhaust gas is made to pass through a turbine, the exhaust pressure is increased to lower the intake efficiency of the engine, resulting in a reduced engine output. Further, the supercharging over entire range of engine operation forces the supercharger, exhaust system and the intake system to work under a permanent additional load. This is quite inconvenient from the view point of durability of these systems and the supercharger.
Turning now to the second-mentioned supercharging system as disclosed in the Japanese Patent Laid-open Publication No. 52-18517, the following disadvantage is pointed out. As stated before, the relief valve for allowing the compressed air to be released into the atmosphere is controlled to open and close solely dependent on the intake pressure, i.e. the discharge pressure of the blower of the supercharger. In other words, the relief valve is made to open at the same intake pressure irrespective of the operating condition of the engine. To explain in more detail, the intake pressure at which the compressed air is released during the throttle-part-open engine operation is equal to that at which the excessive supercharging pressure is released during the throttle-full-open engine operation. For instance, if the maximum allowable supercharging pressure in the throttle-full-open engine operating condition is predetermined to be +180 mmHg (gauge pressure), the relief valve starts release of air when the pressure in the supercharging intake air passage reaches +180 mmHg (gauge pressure). This will mean that the relief valve is kept closed, even in the throttle-part-open engine operating condition, until the pressure of air compressed by the blower of the supercharger reaches +180 mmHg (gauge pressure), with a result that a surging of the gas turbine occurs. The surging would be avoided by allowing the relief valve to open at a lower supercharging intake pressure, i.e., +50 mmHg (gauge pressure). However, this relief valve setting will inconveniently limit the maximum supercharging pressure in the throttle-full-open engine operating condition to a level as low as +50 mmHg (gauge pressure). Thus, the second-mentioned known supercharging system falls short of achieving such a control as to allow the release of compressed air at a low pressure during the throttle-part-open engine operation and to raise the supercharging air pressure in the throttle-full-open engine operation.